The present invention relates to an apparatus for inspecting defects of a mask used for manufacturing semiconductor integrated circuits.
A mask having an IC pattern is used to transfer the IC pattern to a semiconductor wafer in the fabrication process of semiconductor ICs. When contamination and partial cutting of an IC pattern of the mask occur, a desired semiconductor IC cannot be prepared, thus decreasing the manufacturing yield of the semiconductor products. A mask inspection apparatus has been conventionally used to detect defects of the IC pattern. According to this apparatus, the mask is irradiated with light, and an optical signal representing a pattern formed on the mask is detected by an image sensor. An electrical mask pattern signal from the image sensor is compared with a reference signal corresponding to design data (LSI chip data) used at the time of forming the mask pattern in units of pixels (dots) so as to detect defects of the mask pattern. A mask inspection apparatus of this type is disclosed in copending U.S. patent applications Ser. No. 535,334, filed Sept. 23, 1983, now U.S. Pat. No. 4,559,603 and entitled "APPARATUS FOR INSPECTING A CIRCUIT PATTERN DRAWN ON A PHOTOMASK USED IN MANUFACTURING LARGE SCALE INTEGRATED CIRCUITS"; and Ser. No. 629,467, filed July 10, 1984, and entitled "APPARATUS FOR INSPECTING MASK FOR USE IN MANUFACTURING LARGE SCALE INTEGRATED CIRCUITS."
When a photomask pattern is inspected by such an apparatus, an area of the photomask corresponding to an LSI chip pattern, which is formed in accordance with the design data, is inspected. After inspection, the photomask is corrected as required. Then, by means of a step and repeat exposure system (hereinafter referred to as stepper), a photoresist layer on a semiconductor wafer is exposed through the mask pattern so that the mask pattern is transferred onto the photoresist layer. In the stepper, the mask pattern is transferred onto the semiconductor wafer for each chip region. The exposing area of the mask for the semiconductor wafer includes a chip pattern portion (light-transmitting and light-shielding portions) and a peripheral portion (light-shielding portion). When a defect such as a pinhole is formed in the peripheral portion of the chip pattern, the defect of the peripheral portion of the mask is transferred onto an adjacent chip region of the semiconductor wafer, yielding defective chips.
In order to solve this problem, a visual inspection of the mask peripheral area by means of a microscope may be conceived. However, when the mask peripheral area is thus visually inspected, the throughput of the inspection step is remarkably degraded. Namely, the automatic inspection of the chip pattern by means of the mask inspection apparatus and the visual inspection of the peripheral area of the chip pattern must be separately performed. Visual inspection is not more precise than automatic inspection, and the operator may fail to find defects. In addition, new defects may be formed between automatic inspection step and visual inspection step.